Hitherto, various technologies for improving withstand voltage of alumina-containing insulators have been developed.
For example, in Patent Publication 1, there is described a high-insulation, high-alumina porcelain composition using “a mixed raw material powder made up of either at least one additive selected from Y2O3, ZrO2 and La2O3, or a solid solution composite oxide of at least one additive selected from Y2O3, ZrO2 and La2O3, and alumina, and an alumina fine particle powder having a particle size of 0.5 μm or less” (see claim 1 of Patent Publication 1).
Furthermore, “an alumina porcelain” of Patent Publication 2 is described as “comprising an alumina (Al2O3) having an average particle size of 1 μm or less, and at least one of a compound of and a mixture of at least one of yttria (Y2O3), magnesia (MgO), zirconia (ZrO2) and lanthanum oxide (La2O3), and Al2O3, which is formed in the grain boundary phase, and as having a porosity of 6 volume % or less” (see claim 1 of Patent Publication 2).
“A high withstand-voltage, alumina-based sintered body” of Patent Publication 3 is described “in case that the respective contents in terms of oxide of Si component, Ca component and Mg component contained in 100 parts by weight of the alumina-based sintered body are respectively represented by S (unit: parts by weight), C (unit: parts by weight) and M (unit: parts by weight), as that the respective contents of the above three components satisfy the following relation,0.95≧S/(S+C+M)≧0.75and as having at least a mullite (Al6Si2O13) crystalline phase as the crystalline phase” (see claim 1 of Patent Publication 3).
In Patent Publication 4, there is described “a rare earth oxide-alumina-silica sintered body which is characterized by that it is a sintered body of 5-95 wt % rare earth oxide, 94.9-4.9 wt % alumina and 0.1-10 wt % silica and that crystal grain size of this sintered body is 30 μm or less” (see claim 1 of Patent Publication 4).
In Patent Publication 5, there is described “a spark plug insulator which is characterized by that, when the sum of the constituent components is defined as 100 mass %, Al component is 95-99.8 mass %, that a rare earth element and a Si component are contained such that the ratio (RRE/Rsi) of the rare earth element (RRE) to the Si component (Rsi) becomes 0.1-1.0, and that the number of alumina particles existing per 1 mm2 of the section to have the maximum length of 10 μm or greater and an aspect ratio of 3 or greater is less than 10” (see claim 1 of Patent Publication 5).
In Patent Publication 6, there is described “an alumina porcelain composition which is characterized by that it is an alumina porcelain composition containing alumina as a main component, that it comprises a composite sintered body of the alumina as the main component and a composition of at least one element selected from Al, Si, Mg and rare earth elements, and that, when the alumina as the main component is 100 parts by weight, the composition of the at least one element selected from the Al, Si, Mg and rare earth elements is 5 parts by weight or less” (see claim 1 of Patent Publication 6).
A spark plug for internal-combustion engines, which is described in Patent Publication 7, is described as that it suffices that a component to be contained in addition to alumina component is made up of one or at least two selected from Si component, Ca component, Mg component, Ba component and B component, that it suffices to conduct the mixing such that the content of the added element series powder is in a range of 4-7 mass % as the sum of the content of each component in weight in terms of oxide, that it is possible to conduct the mixing such that, as each added element series powder, for example, Si component is in the form of SiO2 powder, Ca component is CaCO3 powder, Mg component is MgO powder, Ba component is BaCO3 powder, and B component is H3BO3 powder (or may be aqueous solution), and that, as each component of Si, Ca, Mg and Ba in the added element series powder, it is also possible to use various inorganic raw material powders such as hydroxides, carbonates, chlorides, sulfates, nitrates and phosphates, in addition to oxides.
By the way, there has been a demand for a spark plug equipped with an insulator that is further superior in withstand voltage characteristics and particularly further superior in withstand voltage characteristics at high temperatures susceptible to insulation breakdown, as compared with the above-mentioned conventional spark plugs.